Urinary tract infection (UTI) is one of the commonest infections in women, occurring in nearly 20% during their lifetime. Research on the epidemiology and etiology of UTI has concentrated on two groups of women: those between 18 and 35 years and those who are elderly and debilitated. In younger women, sexual activity, vaginal spermicides, and, possibly, estrogenic effects promote UTI. Certain blood group types enhance mucosal adherence by bacteria and may predispose to UTI. In older women, general debility, voiding problems, diabetes, and, possibly, estrogen deficiency are risk factors for UTI. Little is known about risk factors for UTI occurring in younger post-menopausal women even though there are 56 million women aged 50 to 75. The main goal of this project is to identify risk factors for UTI in postmenopausal women aged 50 to 75 living in the community. The primary aims will be to learn whether post-menopausal estrogens (topical or oral) or urinary incontinence or increased post-void residual urine alter the risk of UTI. Our secondary aims are to evaluate other potential risk factors including: (1) history of UTI, Current sexual activity, or vaginal spermicides, (2) chronic diseases, e.g., diabetes, or (3) presence of the P1 and Lewis blood-groups or absence of blood group antigens B and AB in secretions. We propose a case-control study of community-dwelling, post-menopausal women. Cases will be 800 women identified through monthly surveillance of Group Health Cooperative of Puget Sound (GHC) outpatient and laboratory databases as having symptomatic, culture-confirmed UTIs. Controls will be 800 randomly selected, age-matched women with no history of a recent UTI. Information about estrogen use and other exposures of interest will be obtained by interview. Post-void residual urine volume and bloodgroup and secretor status will be determined for volunteers at a brief clinic visit. This study addresses an important but neglected area. The large sample size ensures precise estimate of relative odds for UTI associated with several exposures while controlling for the potentially confounding effects of variables such as sexual activity and history of prior UTI. Because several exposures of interest are potentially modifiable, the results of this study should be relevant to clinical practice.